In some applications, it is important to be able to accelerate a projectile along a path to relatively high velocities in short periods of time. For example, accelerations to velocities in the range of 2.5 to 4 kilometers per second within a length of a few meters are often desirable. It is further desirable for such projectile accelerators to be efficient.
Accelerators based on the interaction of magnetic fields with the projectile to be accelerated are well-known. Among others, they have taken the forms of rail guns, coaxial accelerators, and coil-shortening accelerators. Rail guns accelerate a conductive projectile through the interaction of a transverse current passing through the projectile from a pair of conductive rails and a magnetic field created by the current. Their proper operation depends upon the development of reliable sliding contacts between the projectile and the two rails.
Coaxial accelerators operate through the generation of a linear series of coaxial magnetic fields. Each magnetic field in the series is generated by the properly-timed initiation of a circumferential current about the projectile. This magnetic field induces a circumferential current in the projectile, and the current and the magnetic field interact to produce the accelerating force which acts on the projectile. A major problem with coaxial accelerators is that the series of magnetic fields must be properly timed and rapidly energized in order to optimize the acceleration of the projectile.
Coil-shortening accelerators operate under the principle that the magnetic fields generated by two similarly wound coils will attract one another. By attaching one such coil to the projectile, the projectile can be accelerated with respect to the other coil. In order to maintain the magnetic field in both coils simultaneously, it is generally necessary that a sliding contact be made between the two coils. As with rail guns, a major problem with coil-shortening accelerators is the development of reliable sliding contacts. In addition, some embodiments of coil-shortening accelerators require an abrupt change in coil resistivity (for example, a piece of superconducting material going from its superconducting state to its normal state).
A more conventional means of accelerating a projectile is based on chemical reactions. While they are well-developed, such chemical accelerators are either inefficient or have ultimate projectile velocities limited to between 1 and 1.5 kilometers per second. This velocity is inadequate for many applications.
It is therefore desirable to have a projectile accelerator which is highly efficient and capable of accelerating projectiles to velocities at least on the order of 2.5 to 4 kilometers per second.